1. Field of the Invention
The present invention relates to a bidirectional add/drop multiplexer (BADM) and a bidirectional add/drop amplifier (BADA) module. More particularly, it relates to the BADM and the BADA module with a mid-stage device that is shared by the counter-propagating signals in different wavelength bands.
2. Description of the Related Art
A wavelength-division multiplexing (WDM) optical network uses different wavelengths to establish communication channels among the nodes. The network requires add/drop function for receiving/transmitting wanted wavelength signals at each node. Therefore, a WDM add/drop multiplexer (ADM) is an important constituent to realize the WDM optical network. The ADM is especially emphasized in a WDM ring network.
A bidirectional WDM ring network can be realized by using either undirectional ADMs or BADMs. The BADM, which can add/drop WDM signals propagating bidirectionally over a single fiber, reduces the use of fiber infrastructure by a factor two.
FIG. 1 shows an example of such a BADM proposed in a paper of C. H. Kim, Chang-Hee Lee, and Y. C. Chung, xe2x80x9cA novel bidirectional add/drop amplifier (BADA),xe2x80x9d IEEE Photon. Technol. Lett., vol. 10, pp. 1118-1120, 1998. Compared to the conventional BADM, the bidirectional add/drop amplifier (BADA) module of FIG. 1 further provides the function of signal amplification. Namely, the BADA module could not only add/drop but also amplify WDM signals propagating bidirectionally over a single fiber. Especially the BADA module of FIG. 1 was used to accommodate two groups of WDM signals whose wavelength-bands are split according to the their propagation direction.
Meanwhile, in order to increase the capacity and extend the geographical scale of the WDM networks, the following three means are generally used: (1) Means for compensating the chromatic dispersion of the optical fibers. (2) Means for flattening the spectral response of the optical amplifiers. (3) Means for suppressing the accumulation of the amplified spontaneous emission (ASE) noise. The dispersion compensating fiber (DCF) is a typical means for compensating the chromatic dispersion of the optical fibers, the gain flattening filter (GFF) for flattening the spectral response of the optical amplifiers, and the ASE rejection filter for suppressing the accumulation of the ASE noise. These devices are usually used in the mid-stage of the optical amplifiers in order to minimize the optical signal-to-noise ratio degradation.
There are some problems to incorporate the midstage devices with the BADA module of FIG. 1. For example, we should use different DCF for each way signal to prevent the signal degradation induced by the large back reflection of the DCF, which increases the cost of dispersion compensation.
The present invention is contrived in order to solve the above-mentioned problems. It is an object of the present invention to provide a BADM and a BADA module. Wherein, (1) the mid-stage device for increasing the capacity and extending the geographical scale of the networks, including a DCF, a GFF and an ASE rejection filter, is shared by the counter-propagating signals. (2) The counter-propagating signals pass through the shared mid-stage device in opposite direction each other to minimize the signal impairment in it. (3) The relative intensity noise (RIN) induced by the multiple reflection is effectively suppressed.
In order to meet the above object, the present invention includes an Nxc3x97N arrayed waveguide grating router (AWGR), two optical circulators, two wavelength selective couplers, two optical isolators, and a mid-stage device. The mid-stage device comprises a means for compensating the chromatic dispersion of the optical fibers, a means for flattening the spectral response of the optical amplifiers, a means for suppressing the accumulation the ASE noise or a combination of these means.